Industry uses numerous rheology-modifying agents, in particular thickening agents, in varied compositions. These agents are generally chosen so as to be able to thicken a given composition and to confer, on said composition, a specific rheological behavior, for example a certain threshold, a viscoelasticity, a thixotropy, a shear-thinning behavior, a heat-thickening behavior, a newtonian behavior, a non-newtonian behavior, and the like.
Use is frequently made, among thickening agents, of polymers. A great variety of them exist and the choice is generally made of a polymer suited to the composition whose rheology is to be modified, in order to confer the desired properties on it, if appropriate by physical and/or chemical interaction with other compounds. Many polymers are thus used in cosmetic compositions to confer, on the compositions, an appropriate stability and/or a viscosity and a texture which are appreciated by consumers. Some compositions can exhibit a high ionic strength and/or comprise a large amount of salts. In the construction and civil engineering field, attempts are often made to modify the viscosity of compositions or fluids, in particular in underground uses, where the presence of liquids may be a hindrance, in particular in tunnel construction, digging and/or excavation operations. Many fluids are employed in the field of the exploitation of oil and/or gas fields and are either prepared outside the geological formation being exploited or are formed in the formation with a liquid present in the formation. Fluids are thus used in particular during drilling, stimulation and production operations. These fluids very often require a highly specific rheology, with a greater viscosity than that of water. These issues are known in this field. The fluids may exhibit a high ionic strength and/or comprise relatively large amounts of salts; they can in particular be based on seawater or brines present in the geological formations.
There exists a constant need for novel polymers which can vary the rheological properties of different fluids, in particular in the presence of salts and/or at a relatively high ionic strength. It should be noted that it is generally important for the polymers to be stable in the fluid or of controllable stability, that is to say for them not to separate from the other constituents, so that in particular the rheological properties are retained for the desired time. It should also be noted that it is sometimes important for the rheological properties to be obtained under specific conditions, for example under severe conditions of temperature and/or pressure. This may be the case in particular for fluids employed in certain underground operations, for example in the context of the exploitation of oil and/or gas fields. This can also be the case in the context of the use of some industrial manufacturing processes.
Copolymers comprising units comprising a betaine group are known.
The document U.S. Pat. No. 6,346,588 (SKW) describes terpolymers of from 1 to 75 mol % of units deriving from a sulfobetaine, from 5 to 79% of units deriving from acrylamide (AM) and from 20 to 94% of units of hydroxyethyl methacrylate (HEMA) type. All these units are hydrophilic units. The terpolymers exhibit thickening effects on saline media.
The documents U.S. Pat. No. 4,742,135, U.S. Pat. No. 4,650,848 and/or U.S. Pat. No. 4,882,405 (Exxon) describe terpolymers of from 1 to 30 mol % of units deriving from a sulfobetaine, from 70 to 95% of units deriving from acrylamide (AM) and from 0.1 to 5% of hydrophobic units deriving from alkylacrylamide. The polymerization is carried out in the presence of large amounts of surfactant (SDS) not making it possible to obtain groups of the hydrophobic units. Furthermore, the polymerization proves to be difficult to reproduce. The terpolymers exhibit thickening effects on saline media.
The document U.S. Pat. No. 4,788,247 (Exxon) describes terpolymers of from 1 to 30 mol % of units deriving from a sulfobetaine, from 70 to 95% of units deriving from acrylamide (AM) and from 0.1 to 5% of hydrophobic units deriving from an ethoxylated alcohol acrylate. The polymerization is carried out in the presence of large amounts of surfactant (SDS) not making it possible to obtain groups of the hydrophobic units. Furthermore, the polymerization proves to be difficult to reproduce. The terpolymers exhibit thickening effects on saline media.
The documents U.S. Pat. No. 4,607,076 and/or U.S. Pat. No. 5,153,289 (Exxon) describe copolymers of units deriving from a sulfobetaine and of units deriving from vinylpyrrolidone (VP). All these units are hydrophilic units. The copolymers exhibit thickening effects on saline media.
The documents U.S. Pat. No. 4,585,846, U.S. Pat. No. 4,822,847 and/or U.S. Pat. No. 4,708,998 (Exxon) describe copolymers of units deriving from a sulfobetaine having a special structure and of units deriving from vinylpyrrolidone (VP). All these units are hydrophilic units. The copolymers exhibit thickening effects on saline media.
The document U.S. Pat. No. 6,284,854 (Biocompatibles) describes copolymers of from 10 to 50 mol % of units deriving from sulfobetaines and from 50 to 90% of hydrophobic units. These copolymers are used as biocompatible coatings. The polymerizations are carried out in solution, which does not make it possible to obtain groups of the hydrophobic units.
The document “Hydrophobically Modified Acrylamide-Based Polybetaines. I. Synthesis, Characterization and Stimuli-Responsive Solution Behavior”, M. Johnson et al., Journal of Applied Polymer Science, Volume 92, Issue 1, pp. 647-657 (2004), describes terpolymers of units deriving from certain sulfobetaines, of acrylamide and of N-butylphenylacrylamide (BPAM). The terpolymers are prepared by micellar polymerization. The polymerization is carried out in the presence of large amounts of surfactant (SDS) not making it possible to obtain significant groups of the hydrophobic units (value of nH of 2.1).
Micellar radical polymerization processes which make it possible to polymerize, in a hydrophilic macromolecular chain, units deriving from hydrophobic monomers are also known.
The documents U.S. Pat. No. 4,831,092 and/or U.S. Pat. No. 4,835,234 (Exxon) describe a process for the micellar polymerization of certain monomers. The process is carried out batchwise.
The documents EP 1 250 360 and/or U.S. Pat. No. 417,268 (Hercules) describe a process for the micellar polymerization of certain monomers. These documents describe a series of special stages for the preparation of the reaction medium but the polymerization is carried out batchwise.
The document WO 99/03895 (Rhodia) describes a process for the micellar polymerization of certain monomers. The process comprises the prior preparation of a starting charge comprising hydrophilic monomers and micelles of hydrophobic monomers. A solution of initiator and another solution comprising other hydrophilic monomers and micelles of hydrophobic monomers are then added continuously.
The need remains for novel polymers exhibiting modified properties, in particular:                good stability at a relatively high ionic strength, in particular a relatively saline medium,        good thickening power for media comprising a relatively high ionic strength, in particular saline media, indeed even highly saline media,        good retention of the stability and/or thickening at relatively high temperature, and/or        a thickening power at low contents of polymer,        an improved combination and/or compromise of at least two of these properties.        
A need also remains for processes for the preparation of copolymers which are improved and/or which can exhibit at least one of the properties mentioned above.